Prostaglandin (PG) E2 is produced by a variety of cells and tissues and exhibits potent diverse bioactivities. Its production is mediated by three enzymatic reactions involving phospholipase A2 (PLA2), cyclooxygenase (COX), and PGE2 synthase (PGES). In this biosynthetic pathway, arachidonic acid (AA) releases from membrane phospholipids by cytosolic or secretory PLA2 and is converted to prostaglandin H2 (PGH2) by COXs. PGH2 is then isomerized to prostaglandin E2 (PGE2) by terminal PGES enzymes. PGES enzymes, that lie downstream of COXs, occur in three forms in mammalian cells. Among them, the microsomal and membrane-bound synthase (namely mPGES-1) has received much more attention and established as a novel drug target in the areas of inflammation, tumorigenesis, and bone disorders. Hence, mPGES-1 is involved in a number of diseases including arthritis, burn injury and pain diseases, atherosis, cancer, and even the exacerbation of Alzheimer's disease. Recently reported studies have led to the characterization of its inducible distribution, expression, enzymatic kinetics, and biological and pathological functions. The expression of mPGES-1 is up-regulated by pro-inflammatory stimuli and down-regulated by anti-inflammatory glucocorticoids, often in accordance with that of COX-2. The protein mPGES-1 has been identified as the central switch during immune-induced pyresis, and deletion of mPGES-1 would reduce inducible and basal PGE2 production and alter the gastric prostanoid profile. Compared to its up-stream enzymes, inhibition of mPGES-1 does not block normal functions of other PGs and, therefore, lacks the unexpected side effects produced by the inhibition of COXs, making it more attractive for the development of potential therapeutics, especially for the treatment of inflammation-related diseases. However, no clinically useful inhibitor of mPGES-1 has been identified. To date, only two types of compounds, i.e. the COX-2 inhibitor NS-398 and 5-lipoxygenese-activating protein (FLAP) inhibitor MK-886 (see FIG. 9) and similar compounds (see e.g., Riendeau et al., Bioorg. Med. Chem. Lett., 15:3352-3355), have been found to be able to inhibit mPGES-1. None of these compounds is selective for mPGES-1. It is highly desirable to develop more potent and selective inhibitors of mPGES-1 based on the structure and function of the enzyme for development of the next-generation therapeutics.
Initially, mPGES-1 was discovered as recombinant human microsomal glutathione-S-transferase (GST)-1-like 1 (MGST1-L1) and recognized as a member of membrane-associated proteins involved in eicosanoid and glutathione (GSH) metabolism (MAPEG) superfamily. It shows significant homology with other MAPEG proteins, especially with the nearest subfamily member MGST1. Hydropathy analysis suggests that all the MAPEG proteins have similar three-dimensional and membrane-spanning topological properties. Site-directed mutagenesis revealed that R110 has an essential role in the catalytic function of mPGES-1, whereas the mutation on either R51 or R70 did not affect the activity. Unfortunately, further structure-function investigation is restrained by the lack of the detailed three-dimensional structure of this membrane-bound protein, making the structure-based design of drugs targeting mPGES-1 difficult. A two-dimensional (2D) electron projection map (with a resolution of 10 Å) of mPGES-1 revealed a trimer structure (Thoren, et al., J. Biol. Chem. 2003, 278, 22199-22209) which is very similar to that of MGST1, but the resolution of 10 Å is insufficient for the purpose of building a three-dimensional model of mPGES-1.
Accordingly, more precise models of the three-dimensional structure of mPGES-1 are needed so that potent and selective modulators of mPGES-1 activity can be identified.